killbill-memoizeit

diff --git a/invoice/src/main/java/org/killbill/billing/invoice/usage/ContiguousIntervalConsumableUsageInArrear.java b/invoice/src/main/java/org/killbill/billing/invoice/usage/ContiguousIntervalConsumableUsageInArrear.java
index 2898217..7e0d45a 100644
--- a/invoice/src/main/java/org/killbill/billing/invoice/usage/ContiguousIntervalConsumableUsageInArrear.java
+++ b/invoice/src/main/java/org/killbill/billing/invoice/usage/ContiguousIntervalConsumableUsageInArrear.java
@@ -136,7 +136,7 @@ public class ContiguousIntervalConsumableUsageInArrear extends ContiguousInterva
                         resultMap.put(curDetail.getTier(), curDetail);
                     } else {
                         final UsageConsumableInArrearTierUnitDetail perTierDetail = resultMap.get(curDetail.getTier());
-                        perTierDetail.updateQuantityAndAmount(perTierDetail.getQuantity() + curDetail.getQuantity());
+                        perTierDetail.updateQuantityAndAmount(curDetail.getQuantity());
                     }
                 }
             }

diff --git a/invoice/src/main/java/org/killbill/billing/invoice/usage/details/UsageConsumableInArrearTierUnitDetail.java b/invoice/src/main/java/org/killbill/billing/invoice/usage/details/UsageConsumableInArrearTierUnitDetail.java
index 5b275f7..1805cbf 100644
--- a/invoice/src/main/java/org/killbill/billing/invoice/usage/details/UsageConsumableInArrearTierUnitDetail.java
+++ b/invoice/src/main/java/org/killbill/billing/invoice/usage/details/UsageConsumableInArrearTierUnitDetail.java
@@ -71,12 +71,12 @@ public class UsageConsumableInArrearTierUnitDetail extends UsageInArrearTierUnit
         return tierBlockSize;
     }
 
-    public void updateQuantityAndAmount(final Integer targetQuantity) {
-        this.quantity = targetQuantity;
-        this.amount = computeAmount(tierPrice, targetQuantity);
+    public void updateQuantityAndAmount(final Integer addionalQuantity) {
+        this.quantity = quantity + addionalQuantity;
+        this.amount = computeAmount(tierPrice, quantity);
     }
 
     private static BigDecimal computeAmount(final BigDecimal targetTierPrice, final Integer targetQuantity) {
-        return targetTierPrice.multiply(new BigDecimal(targetQuantity));
+        return targetTierPrice.multiply(BigDecimal.valueOf(targetQuantity));
     }
 }